The satisfactory reproduction of a picture requires the transmission of several types of information combined into a single waveform called a composite video signal. The signal is composed of video information and synchronizing information. Composite video describes a signal in which luminance, chrominance, and synchronization information are multiplexed in the frequency, time, and amplitude domain for a single-wire distribution. Luminance is defined as the signal component in color video systems that represents the brightness of the image. Chrominance is defined as the component signal in color video systems that describes color information.
The video signal conveys information concerning the blanking level, the black reference level, average scene brightness level, picture details, and color values. The luminance component of a baseband video signal is unipolar with one direct current (“DC”) level (nominally 0 volts) representing blank, and a second level (nominally +700 mV) representing white. Any level between 0 and 700 mV represents a degree of gray. For NTSC and PAL the color information consisting of two orthogonal color vectors is quadrature amplitude modulated (QAM) onto a common subcarrier forming the chrominance component (chroma) whereas SECAM employs frequency modulation (FM) of line-wise alternating carriers for this purpose. This chrominance component (chroma) is superimposed to luminance to form the composite video signal.
The synchronizing information consists of horizontal and vertical scanning synchronization, and chrominance decoder synchronization. The horizontal and vertical synchronization information is used to align the horizontal and vertical deflection circuits in the receiver. The horizontal sync tells the display where to put the video signal in the left-right dimension, and the vertical sync tells the display where to put the signal in the top-bottom dimension. Synchronization consists of pulses having a specific amplitude, duration, and shape best suited to the task at hand. The synchronizing pulses are unipolar with a reference level of 0 V and a peak negative level of nominally −300 mV.
The video signal waveform, with a nominal peak-to-peak amplitude of 700 mV, and the synchronizing signal waveform with a nominal peak-to-peak amplitude of 300 mV, are added together to form a composite video signal of 1 V peak-to-peak. The synchronizing pulses are placed in parts of the composite signal that do not contain active picture information. These parts are blanked (forced at or below the black level) to render invisible the retrace of scanning beams on a correctly adjusted display.
The standard video signal levels apply to both conventional television scanning standards—National Television System Committee (“NTSC”) and Phase Alternating Line (“PAL”) as well as Séquentiel Couleur à Mémoire (“SECAM”). The U.S standard is NTSC which uses 525 lines at 60 Hz field rate, while PAL and SECAM are predominant in Europe and use 625 lines at 50 Hz field rate. Composite video signals are expressed in IRE units. An IRE unit is defined as one-hundredth of the excursion from the blanking level (0 IRE units) to the reference white level (100 IRE units). A standard 1 V peak-to-peak signal is said to have an amplitude of 140 IRE units (143 for PAL and SECAM) of which 100 IRE units are luminance, and 40 IRE units (43 for PAL and SECAM) are synchronization information. Further discussion of video circuits and signals can be found in the following texts: M. Robin, DIGITAL TELEVISION FUNDAMENTALS, McGraw-Hill (1998); K. Jack, VIDEO DEMYSTIFIED, 2nd Edition, Harris Semiconductor (1996); and A. Inglis, VIDEO ENGINEERING, 2nd Edition, McGraw-Hill (1996), all of which are hereby incorporated by reference. Although different versions of PAL and SECAM are different in terms of chrominance modulation (quadrature amplitude modulation versus FM), RF modulation (negative vs. positive), and sound modulation (FM vs. AM), the luminance component and the synchronization aspects are defined identically. Thus they are commonly referred to hereinafter by the term PAL.
A frame of video is essentially one picture or “still” out of a video stream of pictures. In NTSC, a frame comprises 525 individual scan lines (for PAL 625 lines). For NTSC, after 525 lines have been displayed on the screen, the picture presentation process continues with the next frame of 525 lines. An interlaced TV screen (and only an interlaced scanning system) is made using two fields, each one containing half of the scan lines needed to make one frame. Although in analog terminology, each field is considered to have 262.5 lines, in the digital domain, it is convenient to consider each field comprising a whole number of lines; 263 for the odd field, and 262 for the even field. For NTSC, the lines number 1-263 for the odd field, and 264-525 for the even field. The composite video signal contains a vertical sync pulse which signals the start of the odd and even fields. Two fields comprise a frame. The first 9 lines of both the odd and even fields are vertical sync pulses. For NTSC, 9 lines (7.5 lines for PAL) of both the odd and even fields contain equalization and serration pulses for the purpose of vertical synchronization. Each field is displayed in its entirety—therefore, the odd field is displayed, then the even field, then the odd field, and so on. The vertical scan frequency is chosen so that half of the scanning lines are contained in each field. This causes the first line of alternate fields to begin at the horizontal center of the top line of the picture, and the lines are interleaved between fields. Each field occurs at a rate of 60 Hz for NTSC (50 Hz for PAL) television standards.
The video decoder must lock to the vertical sync and then output a vertical sync for each of the two fields. NTSC consists of 525 horizontal lines per frame or 525 half lines per field; PAL consists of 625 horizontal lines per frame or 625 half lines per field. Odd and even field vertical syncs may be detected by their position with respect to line boundaries; the odd sync switches at a line boundary and the even sync switches at a half line boundary.
Proper vertical lock must be achieved not only for standard video but for non standard video, too. Non-standard video sources, such as VCR, video games, macrovision, and weak noisy signals, present problems to logic that is designed for standard video inputs. In a non-standard video mode, video signals obtained from sources such as the VCR in a trick-mode (rewind, fast forward, and pause modes), and video games, may output frames which do not have the standard number of lines per frame (e.g. 528 lines in a 525-line NTSC standard). Non-standard signals may exhibit a lack of serration pulses which normally indicate the start of the sync pulse. (Vertical sync is identified by broad pulses, which are “serrated” in order for a receiver to maintain horizontal sync, even during the vertical interval). The signal may instead provide one large broad pulse where the serration pulses are normally expected in a standard video signal. The absence of sync level between the end of a broad pulse, and the start of the following sync pulse is called serration. Thus decode from the line counter is not possible. The inability to detect a non-standard signal may result in vertical roll of the picture presented to the viewer.
In a VCR, a horizontal sync jump, which occurs prior to vertical sync due to head switching, makes detection of odd or even field vertical syncs difficult. Due to confusion, odd and even fields may constantly switch in the output picture. Vertical lock may also be difficult to achieve with weak noisy signals which have a low signal to noise ratio. This non-standard mode must be detected and a vertical sync output when a sync is detected at the input.
Another problem arises when there is no video input present. In such a situation, it is still desirable to output a sequence of vertical syncs in a free running mode so that a blank screen is displayed on the monitor. Automatic detection of these modes is a preferred feature which yields a stable blank picture.
Previous implementations of vertical sync detectors for video decoders have used half line accumulators of the input pixels to detect odd and even field vertical syncs. The half line sums are compared against an adaptive threshold based on the minimum half line sum detected over a frame. If a sum falls below the threshold then a vertical sync is detected; an odd or even field is based on the position with respect to the line boundary. If the odd field vertical sync is detected, a line counter is reset to one else the counter increments. If two successive odd field syncs are detected, as in the case of an even number of lines per frame, then the line counter resets on every other odd sync. For a standard video, the odd and even field vertical sync outputs are decoded from the line count. For non standard video, the output is simply generated when a sync at the input is detected. When no video input signal is present, the line counter simply free runs counting from one up to the lines per frame for that standard and then resets to one. A state machine controls the different modes of operation. Consequently, previous implementations of the vertical sync detector could only replenish any eventual missing or undetectable vertical syncs (due to excessive noise, VCR head switch, etc.) at offsets of integer multiples of standard half line increments (525 for NTSC and 625 for PAL). They were incapable of automatically adapting to non standard number of half lines per field.
It is therefore desirable for the present invention to overcome the limitations described above that are involyed in a vertical sync PLL for a video decoder.